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3 Answers
3

I guess that the most reliable way is decide from the context. But at least in the Tokyo dialect and other dialects with the same accent pattern, they have different accents.

I think that もう meaning “already” is pronounced as HL (where H is high and L is low). Therefore もうにほん becomes HLHLL.

もう meaning “additional” is pronounced as LH. Therefore もうにほん becomes LHHLL. This can be confirmed by the Daijirin dictionary. This meaning is shown as 1-[3], and the small “0” at the beginning of this meaning means that the “standard” accent is LH.

I think the OP's grammar in the second example is off. Thx for the looking up the pronunciation guide.
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crunchytJun 4 '11 at 5:34

@crunchyt, When someone suggest you another(3rd) bottle, after you drank two additional bottles than original plan, he may use "もう二本飲みましたよ", which is valid, IMHO.
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YOUJun 4 '11 at 5:40

thanks for the pronunciation hints, I've never realized till now but it's true! :)
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UbertoJun 4 '11 at 9:29

Seeing istrasci’s answer, I realized that I had misread the question. I thought that the question was about how to distinguish the two meanings, not how to differentiate between them.
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Tsuyoshi ItoJun 5 '11 at 2:33

Thanks for the answer. So it's from the accent. Now that you mentioned it I think I remember hearing the two different accents of もう in some anime I watched (although the characters did not actually say the two example sentences ...)
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LukmanJun 5 '11 at 14:51

Well, もう二本飲みました can mean “I drank two more bottles,” depending on the context. For example, if a doctor asked me how much beer I drank last week, I could answer 月曜に中瓶で一本飲んで、水曜にもう二本飲みました。 (I drank one middle-sized bottle on Monday, and two more on Wednesday.)
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Tsuyoshi ItoJun 4 '11 at 5:48

True, thus is the limitation of a single sentence example out of context. :D
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crunchytJun 20 '11 at 1:22